"It was the final unanswered question about Windows 7. But now, thanks to numerous reader reports and my own hands-on experience, I'm can now report that Microsoft is still making it difficult to clean install Windows 7 with Upgrade media. But fear not, there is some good news. While you can't simply use Upgrade media to do a clean install of Windows 7 on an unused PC with a blank hard drive, the workaround this time is easier than ever. Assuming you know the trick.

Remember how this used to work? In older versions of Windows, Microsoft would actually prompt you to insert an install floppy or CD from a previous Windows version, to prove that you qualified for the upgrade version. But beginning with Windows XP, PC makers were able to dramatically change the Windows install disc, so much so that, in some cases, those discs weren't even identifiable as valid install media to Windows Setup. Clearly a different system was required."